Any water system, with the right environmental conditions, could be a source for legionella bacteria growth. There is a reasonably foreseeable legionella risk if your water system:
The most common sources of legionella are in man-made water systems including:
There are also a number of other potential risk systems that may pose a risk to exposure to legionella, e.g. humidifiers, air washers, emergency showers, indoor ornamental fountains etc.
If you are an employer, or someone in control of premises (e.g. landlord), you have a duty to understand and manage legionella risks. Further information is available in 'What you must do'. All systems require a risk assessment but not all systems will require elaborate control measures. A simple risk assessment may show that the risks are low and being properly managed to comply with the law. If such cases, your assessment may be complete and you may not need to take any further action, but it is important to review your assessment regularly in case anything changes in your system.